Amplitude demodulation of high-frequency radio signals received by an antenna in a radio receiver is conventionally performed using a mixer or diode demodulator which operates in a nonlinear regime. The mixer or diode demodulator has an electrical output response Φout that can be generally characterized in terms of its input signal Φin by a Taylor series expansion:
      Φ    out    =            a      0        +                  a        1            ⁢              Φ        in              +                  a        2            ⁢              Φ        in        2              +                  a        3            ⁢              Φ        in        3              +                  a        4            ⁢              Φ        in        4              +    …  where the input signal Φin can be amplitude modulated and characterized as a sinusoidal carrier having a time varying multiplying factor which contains information to be transmitted:Φin=b0(1+m0·m(t))cos(ωct)where ωc is the carrier frequency expressed in radians per second and is related to the carrier frequency fc expressed in Hertz by ωc=2πfc.
When this amplitude-modulated (AM) input signal Φin is passed through a conventional diode demodulator (also termed a “square-law” detector), the Taylor series expansion above is invoked, but only the squared term is useful to provide a demodulated output signal for recovery of the AM information. The squared term appears as:
      Φ    out    =            a      2        ⁢                                        b            0            2                    2                ⁡                  [                      1            +                          cos              ⁡                              (                                  2                  ⁢                                      ω                    c                                    ⁢                  t                                )                                              ]                    ·              [                  1          +                      2            ⁢                          m              ⁡                              (                t                )                                              +                                    m              2                        ⁡                          (              t              )                                      ]            In the above equation, the cos(2ωct) term is filtered out by an RC filter formed by the “square-law” detector's output resistance and video capacitance, or by the use of an external filter. The resulting “square-law” detector output is then given by:
      Φ    out    =            a      2        ⁢                            b          0          2                2            ⁡              [                  1          +                      2            ⁢                          m              ⁡                              (                t                )                                              +                                    m              2                        ⁡                          (              t              )                                      ]            
The efficiencies of this “square-law” detection process are twofold. First, the coefficient a2 reduces the available power in the output signal Φout from the “square-law” detector to a small fraction of the power in the input signal Φin. Second, filtering out the cos(2ωct) term becomes increasingly difficult as the modulation bandwidth of the amplitude-modulated m(t) signal containing the information being transmitted becomes a significant fraction of the carrier frequency ωc. For ultra-wide-band (UWB) signals the modulation bandwidth must be at least 25% of the carrier frequency ωc to satisfy Federal Communications Commission (FCC) requirements.
The above analysis shows that conventional diode demodulators based on “square-law” detection are inefficient. In a conventional passive diode demodulator and filter combination, the insertion loss can be about −40 dB, or even lower. For a powered diode demodulator, the insertion loss can be reduced; but the overall power required including that for powering the diode demodulator can result in a ratio of receiver output power to total power which is on the order of −50 dB.
Similar losses can occur when a mixer is used for demodulation. In this case, the input signal Φin is multiplied by a local oscillator signal cos(ωLOt) in the mixer to provide an output signal Φout given by:Φout=b0[1+m(t)cos(ωct)]·cos(ωLOt)The mixer generates up-converted and down-converted signal components in the output signal Φout as follows:
      Φ    out    =            b      0        ⁡          [                        cos          ⁢                      (                                          ω                LO                            ⁢              t                        )                          +                                            m              ⁡                              (                t                )                                      2                    ⁢                      {                                          cos                ⁡                                  [                                                            (                                                                        ω                          LO                                                -                                                  ω                          c                                                                    )                                        ⁢                    t                                    ]                                            +                              cos                ⁡                                  [                                                            (                                                                        ω                          LO                                                +                                                  ω                          c                                                                    )                                        ⁢                    t                                    ]                                                      }                              ]      
When the mixer frequency ωLO is equal to the carrier frequency ωc, the output signal Φout is demodulated as:
      Φ    out    =                    b        0            2        ⁢          m      ⁡              (        t        )            The output signal Φout is reduced by at least 6 dB in this process, assuming perfect filtering. If the modulation bandwidth approaches the bandwidth of the input signal Φin, the demodulated output signal Φout will be attenuated even more. Additionally, most receivers use multiple mixer and filter stages which can also reduce the output signal Φout. Furthermore, the above analysis assumes perfect multiplication; whereas, in practice, the multiplication is accomplished by means of a nonlinear interaction induced by using the local oscillator to drive an active device into a nonlinear regime. This produces the same Taylor series expansion described above for the diode demodulator based on “square-law” detection so that the efficiency of mixing can be comparable to that of the diode demodulator particularly for low-level input signals.
Other conventional demodulation schemes are essentially more complicated than those described above. A phase-locked loop demodulator behaves like a mixer in terms of its efficiency. Radio-frequency (rf) power detectors are generally diodes or linear conversion sensors and provide an insertion loss which is similar to the diode demodulator.
In conventional demodulators, the desired output signal containing information which has been impressed upon a carrier signal using amplitude modulation is only a small fraction of the receiver input signal so that the signal detection process is inefficient. Additionally, the problem of separating an AM information signal m(t) from the carrier frequency ωc becomes very difficult and very inefficient as the frequency of the information signal m(t) approaches the carrier frequency ωc. For a UWB signal with a 25% modulation bandwidth, the detection problem is profoundly inefficient. What is needed is a more efficient type of demodulator which can be used to demodulate signals over a wide frequency range up to several GigaHertz (GHz) or more.
The present invention is a pyroelectric demodulator which operates passively to demodulate an AM electrical input signal to remove modulation at the carrier frequency ωc and generate an electrical output signal containing the AM information.
These and other advantages of the present invention will become evident to those skilled in the art.